Measurement of the shape and deformation of an object is extremely important for evaluating mechanical properties, structural instability, crack propagation, and residual stress. With miniaturization and integration of products, measurement of deformation at millimeter, micron, and nanoscale levels has attracted much attention in the field of materials science.
Techniques generally used for measurement of the three-dimensional shape and measurement of minute deformation of an object at present include the digital image correlation (DIC) method, electronic speckle pattern interferometry (ESPI), geometric phase analysis (GPA), the phase analysis methods (for example, Fourier transformation, the phase shifting method, the spatiotemporal phase shifting method, and the like), and the moiré methods, and the like as optical full-field measurement methods.
Although DIC has high measurement accuracy, it is disadvantageous in that the deformation measurement is easily influenced by the disturbance of noise. ESPI is significantly sensitive to vibration. Since the specimen grating should be directly observed in GPA, the field of view is narrower than that in the scanning moiré method. The Fourier transformation method is another method for analyzing small-scale deformation in a plane, but the field of view is also narrower because the specimen grating to be used should be observed (the width of the field of view is usually less than 100 times the pitch of a grating).
The moiré methods are effective for performing non-destructive measurement of the deformation distribution from changes in moiré fringes before and after deformation. Among various moiré methods, the microscope moiré method, the CCD or CMOS moiré method (hereinafter, referred to as the CCD moiré method), moiré interferometry, the sampling moiré method, the digital moiré method, and the overlapped (geometric) moiré method have attracted much attention and are applied to deformation measurement of various materials and structures.
The microscope moiré method and moiré interferometry are suitable for measurement of micron/nanoscale deformation using a microscope. The CCD moiré method, the sampling moiré method, the digital moiré method, and the overlapped moiré method can be used to measure nanoscale to meter scale deformation in the case in which a grating image can be acquired.
The moiré methods have been applied to measurement of strains of materials such as metals, polymer, and composite materials and deformation of structures such as bridges and buildings. In addition, it is possible to drastically improve measurement accuracy of deformation measurement by analyzing the phase of moiré fringes from a grating image.
Phase analysis of a fringe grating (fringes or a grating, hereinafter, simply referred to as fringes) image include a phase shifting method for analyzing a phase using a plurality of phase-shifted fringe images as shown in FIG. 1, the sampling moiré method for analyzing a phase using only one fringe image, and the spatiotemporal phase shifting method in which a phase shifting method and the sampling moiré method are combined.
In the phase shifting method and the spatiotemporal phase shifting method, it is necessary to capture a plurality of fringe images, and thus the methods cannot be applied to dynamic measurement. The sampling moiré method can analyze a phase from one fringe image and can be applied to dynamic measurement, but there is a problem in that it is impossible to perform high-accuracy measurement in an environment with much noise. A high-accuracy three-dimensional shape and deformation measurement method performed by capturing one fringe image is desired.